Bronze powder and method of producing the same



amount of suitable lubricant may be present.

Patented Mar. 12, 1940 UNITED STATES PATENT OFFICE BRONZE POWDER AND METHOD OF PRODUCING THE SAME No Drawing.

Application March 24, 1937,

Serial No. 132,807

3 Claims.

This invention vrelates to the production of bronze powders, and specifically to the production of dry bronze powders from paste.

Bronze powders made from aluminum and other metals have been widely used as pigments and for other purposes. These-powders are available commercially in dry form and in paste form, and have: been produced by a variety of methods. Generally, however, they are pro duced either by stamping or by ball milling small pieces of the selected metal. The stamping process is usually carried out with the metal in a substantially dry condition, although a small In the ball milling process, however, the metal particles are mixed with lubricating and leafing agents, and with mineral spirits or some other suitable volatile vehicle supplied in sufficient quantity to maintain the metal sludge in a. fluid condition while it is subjected to the disintegrating action of steel balls within a rotating mill. The continued action of the balls on the metal particles serves to reduce them to very tiny flakes. During the operation, the flakes become covered with a film of the leafing agent in such a. way that they leaf readily when subsequently mixed in a suitable oil or varnish vehicle. Upon completion of the stamping action of the ball mill,

, the flaked metal sludge is removed from the mill and the excess liquid removed from the mixture to adjust the metal content, thereby giving a homogeneous paste product suitable for use in paint. If a dry product is desired, the liquid portion'of the paste is removed by evaporation.

I have discovered that a superior powder may be made from this paste by subjecting the powder obtained after evaporation of the solvent, which is today generally known as dry powder, to a further mechanical treatment commonly referred to in the art as polishing. This operation preferably takes place within a cylindrical drum having a series of rotating brushes bearing lightly against'the surface of the drum. Polishing has heretofore been practiced for the purpose of conferring leafing power upon a powder made by a dry process, and performs its function by smoothing out the metal flakes and coating them with a thin film of lubricant. Heretofore, however, it was considered unnecessary to perform any further operation on powder produced from paste as above described.

As hereinabove stated, the polishing of dry stamped powder served the purpose of imparting to the powder a satisfactory leafing power, and was done primarily for this purpose. As distinguished from the common practice of polishing powder produced by stamping, I have taken the powder produced in the ball mill in paste form, and while perhaps having processed it somewhat similarly so far as using heretofore known equipment is concerned, I have obtained a new result in that a powder of equal leafing power but greatly increased covering area has been produced. The leafing power remains substantially the same, but the covering area has in most cases been doubled and the apparent density substantially reduced.

In one test demonstrating the improved results obtained by my process, the starting material was dry aluminum bronze powder obtained by evaporating the solvent from paste produced by ball milling, as described hereinabove. This powder had a leafing power of 65 and a covering area of 10,586 sq. cm. per gram. This is a relatively high covering area, being more than double that of the Standard Varnish grade of aluminum bronze powder, which has had the largest use of any grade. However, after polishing this dry powder in accordance with my invention for a period of about sixteen hours, its covering area was increased to 20,560 sq. cm. per gram, or nearly doubled. In this manner, I have produced for the first time, a dry aluminum bronze powder having a covering area in excess of 20,000 sq. cms. per gram of powder. Such a powder, because of the fineness of flake, gives an exceptionally smooth paint film and has found many applications. The procedure for determining the covering area of a flaked powder is that of Edwards and Mason, described in an article entitled Covering Capacity (on Water) of Aluminum Bronze Powder, which was published in Industrial and Engineering Chemistry, analytical edition, volume 6, page 159 (1934).

In a preferred embodiment of my invention, the ball mill sludge produced as described hereinabove, which comprises a mixture of the metal flakes, mineral spirits, and lubricating agent, is put into a filter press to remove the excess liquid. The partially dried filter cake is then charged into a drying drum, placed under vaccum, and heated to a temperature ranging preferably between about 150 and 200 F. The drying drum is equipped with a suitable agitator for breaking up the filter cake. The liquid remaining in the filter cake is vaporized and drawn off through a filter, and thence to a condenser for collection and reuse. The powder which remains in' the drier may be mixed with a paint vehicle, and makes a good appearing paint of satisfactory scribed, for example.

leafing characteristics. Prior to my invention, this was the only type of drypowder produced from paste products.

I have discovered, however, that some of the flakes, which are apparently stuck together forming agglomerates in powder of this form, can be broken up or separated by a suitable polishing treatment such as that previously de- By applying such a. treatment, I have substantially increased the covering area of the powder. I have accomplished this by placing th dry powder in a polishing and subjecting the flakes to the separating action of the brushes for a period of between 10 and 1.6 hours. While the addition of lubricant is unnecessary, it has been found desirable for some purposes to add about 1 per cent stearic acid to the powder during the polishing operation.

I claim:

1.1'n that method of producing aluminum powders of high covering power which as flaking, aluminum in the presence of is a agent and a volatile liquid to produce sludge containing aluminum flakes. the steps surprising removing the volatile liquid from said *ludge and subjecting the resulting dried flakes and agglcmerates of said flakes to acontinuous brushing action to separate flakes forming said agglomerates until the covering area of the aluminum bronze powder is materially increased.

2. In that method of producing aluminum bronze powders of high covering power which comprises flaking aluminum in the presence of a leafing agent and a volatile liquid to produce a sludge containing aluminum flakes, the steps comprising evaporating the liquid from said sludge under vacuum and subjecting the resulting dried flakes and agglomerates of said flakes to a continuous brushing action to separate flakes forming said agglomerates until the covering area of the aluminum bronze powder is materially increased.

3. A new article of manufacture comprising an aluminum bronze powder having a covering area in excess of 20,000 square centimeters per gram such as is produced by flaking aluminum in the presence of a leafing agent and a volatile liquid to produce a sludge containing aluminum flakes, and subsequently removing the liquid from said sludge and subjecting the resulting dried flakes and agglomerates of said flakes to a continuous brushing action to separate flakes forming said agglomerates until the desired covering area of -16 aluminum bronze powder is obtained.

FURMAN C. ARTHUR. 

